G.R.P. laminates (sometimes referred to as F.R.P.--fibre reinforced plastic laminates) have a high tensile strength to weight ratio but lack adequate rigidity. Therefore G.R.P. articles are often designed with corrugations, compound curves or ribs as an integral part of the finished product in an effort to improve stiffness. However this is frequently not feasible or desirable. The simplest method to overcome the lack of stiffness is to increase laminate thickness by adding extra layers of the same material to form a solid structure. This method is however expensive and adds extra weight to the finished product.
Another way to improve stiffness of a laminate is to use sandwich core construction. The principle is to divide the laminate into two skins which are positioned apart by adding a core material between them. To be effective the core must form a link between the two skins. This forms a much stiffer laminate than if the two skins are side by side in a single laminate. Many different materials are used for cores in various forms. These can be wood, plastic foams, non-woven fabrics, woven 3 dimensional fabrics, cardboard etc. or preformed honeycombs made from paper, plastic, metal and other materials.
Usually the cores are added during the laminating process. Typically the outer skin of the laminate is first laid up in a mould. The core material is then bonded to this, often by setting or bedding it in a layer of wet resin or adhesive. The second or inner skin is then bonded by laminating over the core material.
Occasionally in the G.R.P. industry, the two skins can be moulded separately. The core is then bonded and/or mechanically fastened in place between the two skins.
The cores add varying degrees of structural strength to a laminate in addition to spacing the two skins apart. For the sandwich construction to be effective, there has to be at least a certain degree of structural support provided by the core material. The design aspects of core construction are quite complex but generally the core must be strong enough to hold the two skins in position relative to each other. The core material must have adequate structural strength and be bonded well to the G.R.P. skins so that the sandwich core laminate acts as a single structural unit.
There are several problems associated with sandwich laminates. One of these is low shear strength, especially when using low density core materials that have little structural strength.
Another problem is in fastening other components or fittings to the sandwich laminate. This can cause the two skins to pull apart (delaminate) from the core or squeeze together and crush the core material. There are various methods to overcome this problem which can be costly and/or time consuming.
Not only do core materials add stiffness to a laminate, they also improve noise and thermal insulation as well as provide buoyancy in certain instances.
In more recent times environmental groups have put pressure on the GRP industry to reduce or eliminate solvent emissions during production of laminates. This has forced GRP moulders in certain countries to change production methods from the usual open moulded, hand lay-up technique to an automated or semi-automated closed mould system. Closed mould procedures entail placing dry glassfibre reinforcements between a male and female mould. Resin is then fed under pressure and/or vacuum into the cavity between the two moulds. Once the cavity is full and the reinforcement has been saturated the moulding is allowed to cure before stripping and repeating the process. Sandwich cores used in this process are usually specialised, expensive and consume more resin than their open moulded equivalents resulting in heavier laminates.
The Desirable Properties of a Sandwich Core are Described as Follows:
1. Reduce cost and weight whilst maintaining or improving stiffness when compared to a conventional solid laminate of equivalent thickness. PA0 2. Bond strength between outer skins and sandwich core must be capable of transfering shear forces from the skins to the core without any sign of parting or breaking away at the interface. PA0 3. The integral strength of the core should readily withstand the designed end-use stresses and strains i.e.: The material from which the sandwich core is made should not crack, crumble or breakdown within itself under load. PA0 4. The core should be resistant to crushing. PA0 5. The core should support the outer skins in such a way that it prevents or limits buckling under load. PA0 6. The core should be resistant to fatigue. PA0 7. Be stable and compatible with other materials used in the construction of the laminate. PA0 8. Readily drape over simple and compound curved shapes. PA0 9. Have no detrimental effect on the aesthetic appearance on the surface of the product. PA0 10. Core application should not be complicated or time consuming. PA0 11. Be resistant to water and/or chemical degradation in the event of damage to an outer skin. PA0 12. Have good acoustic and thermal insulation properties as well as remain stable at elevated temperatures. PA0 13. Be compatible with existing hand and mechanical processing techniques. PA0 14. Tailoring should not be difficult or require specialised equipment.
Few, if any, existing sandwich core materials can comply with all the above requirements.
Prior Art Arrangements
Core Materials Typically Used
1) Non-Wovens
A foam is introduced into a non-woven material which is saturated in situ or pre-saturated with a liquid resin which then cures to its moulded shape. The sheet of non-woven can be perforated to assist with absorption of the resin. Alternatively, for closed mould applications, the material can be lightly embossed with a honeycomb type pattern which leaves a network of small channels across the top and bottom surfaces of the non-woven material through which resin flows in order to speed up the absorption process. This system is user friendly but costly and heavy by comparison to other available technologies.
2) Microballoon or Microsphere Paste
Microscopic hollow balloons made from glass, ceramic, plastic etc are blended with a resin to form a lightweight paste (sometimes referred to as a polymer concrete) that can be troweled in place. This system is fairly easy to use and forms an excellent core material. It is however expensive and difficult to produce a core of uniform thickness and therefore stiffness.
3) Foamed Plastic
Polystyrene, polyurethane and polyvinyl chloride (PVC) in foamed form are frequently used as core materials and are available in various densities and thicknesses. Generally the higher the density, the stronger the foam with a corresponding increase in price.
Polystyrene foam is sometimes used because of its low cost but is subject to chemical attack from a wide range of solvents making its unsuitable for most applications unless it is protected with a suitable barrier coat. It is not viable to use this product in most instances. Polyurethane foam is frequently used as a sandwich core material or a forming core where stiffening ribs are bonded onto laminates. This material also has excellent insulation properties. However the product is friable (apt to crumble) and can break down within itself under cyclic loading.
Polystyrene and polyurethane foams are usually supplied in pre-cured blocks or sheets. In this form they are obviously limited to flat or almost flat laminates. Polyurethane can be foamed in-situ between two pre-moulded laminates. This system has its limitations and is not widely used. Polystyrene and polyurethane foams both absorb water in the event of damage of the protective outer skins.
PVC foam is a closed cell material which is not absorbent, is considerably more expensive than polystyrene and polyurethane but offers many advantages that are important for certain applications. Like both previous foams mentioned it will not readily drape over complex shapes. To overcome this problem the PVC foam is cut into small blocks and bonded on one surface to a light, open weave fabric. These sheets of individual blocks can then be draped over gentle curves.
One of the difficulties of all pre-cured plastic foam cores is to ensure 100% contact with the first laminate to which the foam is bonded, it is very difficult to detect air voids trapped at the interface of the bonding surfaces. Sometimes a vacuum bag (or weight) is used to solve the problem. Several proprietary adhesives have been developed to improve the bond between the foam core and outer skins. When the foam is supplied in its drapable block form, the adhesive will migrate through the vertical cuts between each block thereby eliminate any voids and improve the integrity of the laminate.
End Grain Balsa Wood
Sheets of end-grain balsa wood are sometimes used as a core material for sandwich construction. The end-grain structure is used so that the resin or adhesive can penetrate the balsa and therefore improve adhesion to both outer skins. Like PVC foams, end grain balsa is available in blocks held together in sheet form with a gauzelike backing to improve drapability. Although balsa cores have many excellent properties, it absorbs water and is expensive.
Honeycomb Sandwich Cores
There are several pre-formed honeycomb cell core materials available to the GRP industry. One of these is a special honeycomb material made from a treated paper. It is used to form a honeycomb structure that produces an exceptionally light core between two layers of fibreglass. The honeycomb cell paper contours to compound curves.
Honeycomb cores can also be formed from metal, plastic and other materials. These cores are difficult to fabricate and bond to fibreglass laminates. While honeycomb cores find some use in manufactured products, they are not generally used except where exceptional weight savings are critical typically in aerospace and aircraft applications. Most are limited to pre-formed flat panels and cannot be moulded as readily (or at all) as other available core products. In addition to these difficulties, honeycomb cores suffer from delamination problems primarily due to the very limited bonding interface between the tin vertical cell walls and the outer skins. Another problem is that the usually thin honeycomb cells walls have limited resistance to crushing. For this reason a plastic foam can be used to fill each cell thereby supporting the cell walls. This filled cell system increases its resistance to crushing and increases the surface area to which the outer skins are bonded. Of all core materials available, honeycombs generally fall into the most expensive and problematic category hence their limited use.
Three examples of honeycomb cores have been located n an international novelty search.
In EPO 0 628 406 A2 a preformed structure 3 is joined to a surface 1 by means of foaming adhesive 2 to provide a strength for the surface when subjected to localised compressive loads.
U.S. Pat. No. 4,859,517 has a core 12 of pre-manufactured honeycomb construction and skin panels 14 and 16 are adhered thereto, one of the panels being porous and therefore somewhat flexible so that bending of the honeycomb can take place before the adhesive bonds fail.
French Patent 2 171 949 relates to the use of a pre-manufactured honeycomb core to which extends a short distance into the honeycomb from each side.
Other Core Materials and Systems
The aforementioned cores materials and systems are the most commonly used but there are many other possibilities provided they are compatible with the resins and chemicals used during the manufacture of a laminate. In one of the earlier sandwich core systems, a layer of adhesive paste was applied to the first skin. Before the paste was allowed to harden, a series of small, absorbent blocks were pressed individually into the adhesive forming a mosaic of blocks over the required area. During this process the adhesive paste was forced to rise around each block thereby sealing all the gaps between each individual "cell". The second skin was then laminated over the core which bonded well to both the adhesive and individual blocks. The system resulted in an extremely rigid laminate albeit heavy, labour intensive and expensive.
It is an object of the present invention to provide a method of forming an inexpensive and lightweight sandwich core which has adequate strength; and which meets all or most the requirements listed under the heading "Desirable properties of a sandwich core mdterial".